Resonator filter structure having equal resonance frequencies

ABSTRACT

The invention relates to a resonator filter structure ( 10 ) for radio frequency (RF) filters, especially a bulk acoustic wave (BAW) filter structure. According to the invention, a resonator filter structure ( 10 ) is constructed with a BAW lattice filter section ( 20 ), in which all of the BAW resonator elements ( 20 - 1, 20 - 2, 20 - 3, 20 - 4 ) within the BAW lattice filter section ( 20 ) have substantially equal resonance frequencies. According to the invention, there are parallel capacitances ( 30 - 1, 30 - 2 ) connected in parallel to the BAW resonators ( 20 - 2, 20 - 3 ) of one branch type of the BAW lattice filter section ( 20 ). Thus, anti-resonance frequency of the respective BAW resonator ( 20 - 2, 20 - 3 ) is tuned. That results in a very narrow passband which corresponds approximately to the difference in anti-resonance frequencies between diagonal and horizontal branches of the lattice filter section ( 20 ). The parallel capacitances ( 30 - 1, 30 - 2 ) are used to tune the bandwidth: the smaller the capacitance, the smaller the bandwidth. Moreover, due to the lattice structure at one port of the resonator filter signal guidance will be balanced while at the other port signal guidance can be unbalanced or balanced according to the application needs.

The present invention relates to a resonator filter structure accordingto the preamble of claim 1.

More specifically, the invention relates to a resonator filterstructure, in particular a radio frequency (RF) filter structure,arranged for providing a passband which can be defined by frequencies asa center frequency f_(C), a lower cut off frequency f_(L), a upper cutoff frequency f_(U) comprising between an input port and an output portat least a lattice type filter section having two lattice branch typesbeing a lattice branch and a series branch wherein resonator elementsare arranged in said series branches as series arms having a resonancefrequency f_(X1R) and an anti-resonance frequency f_(X1A) and whereinresonator elements are arranged in said lattice branches as lattice armshaving a resonance frequency f_(X2R) and a anti-resonance frequencyf_(X2A).

The development of mobile telecommunications continues towards eversmaller and increasingly complicated handheld units. This leads toincreasing requirements on the miniaturization of the components andstructures used in the mobile communication means. This concerns radiofrequency (RF) filter structures, which despite the increasingminiaturization should be able to withstand considerable power levels,should have very steep passband edges and low losses, and desirablyshould provide a narrow passband. Due to use of high frequencies in therange of GHz special circuit elements for building RF filter structuresare required and high frequency related concerns have to be dealt with.

Accordingly, it is known to use mechanical resonator characteristics infilter circuits for electrical signals. These resonators can be dividedinto two classes that are derived from the utilized kind of mechanicalvibration. In a first case, surface acoustic vibration modes of a solidsurface are utilized, in which modes the vibration is confined to thesurface of the solid, decaying quickly away from the surface. In otherwords, a surface acoustic wave is travelling on the surface of the solidmaterial, wherein the mechanical or acoustic waves, respectively, arecoupled in and out via applicable formed electrical connections thatcause a frequency selective behavior. Due to the used surface acousticwaves such elements are called Surface Acoustic Wave (SAW) filters orSAW resonators. A SAW resonator typically comprises a piezoelectricsolid and two interdigitated structures as electrodes. Various circuitsas filters or oscillators containing resonator elements are producedwith SAW resonators, which have the advantage of very small size, butunfortunately a weakness in withstanding high power levels.

In the second case, a mechanical vibration of a bulk material is usedwhich is sandwiched between at least two electrodes for electricalconnection. Typically the bulk material is a single piezoelectric layer(piezo) disposed between the two electrodes. When alternating electricalpotential is applied across the metal-piezo-metal sandwich, the entirebulk material expands and contracts, creating the mechanical vibration.This vibration is in the bulk of the material, as opposed to beingconfined to the surface, as is the case for SAWs. Therefore, suchelements are called Bulk Acoustic Wave (BAW) resonators. BAW resonatorsare often employed in bandpass filters having various topologies.Further known BAW resonator elements are thin film bulk acousticresonators, so called FBARs, which are created using a thin filmsemiconductor process to build the metal-piezo-metal sandwich in air incontrast to the afore-mentioned BAWs, which are usually solidly mountedto a substrate.

The electrical behavior of a SAW resonator or a BAW resonator is quiteaccurately characterized by the equivalent circuit, which is shown inthe accompanying FIG. 2. In FIG. 2 there is a branch comprising a seriescombination of an equivalent inductance Ls, an equivalent capacitanceCs, and an equivalent resistance Rs. Ls and Cs are the motionalinductance and capacitance respectively and Rs represents the acousticlosses of the resonator. These series elements are connected in parallelto a capacitance Cp that follows from the dielectric properties of thepiezoelectric material. Therefore, each SAW or BAW resonator comprisestwo characteristic resonance frequencies, which is a series resonancefrequency and a parallel resonance frequency. The first is mostly calledresonance frequency f_(R) and the second is also known as anti-resonancefrequency f_(A).

Circuits comprising BAW or SAW elements in general are better understoodin view of above-introduced element equivalent circuit. The seriesresonance of the individual resonator element is caused by theequivalent inductance Ls and the equivalent capacitance Cs. Atfrequencies that are lower than the series resonance frequency, theimpedance of the resonator element is capacitive. At frequencies higherthan the series resonance frequency of the resonator element, but whichare lower than the parallel resonance frequency of the resonatorelement, caused by the parallel capacitance Cp, the impedance of theresonator element is inductive. Also, at higher frequencies than theparallel resonance frequency impedance of the resonator element is againcapacitive.

As to the impedance characteristic of the resonator element with respectto signal frequency, at the (series) resonance frequency f_(R) of theresonator element, the impedance of the resonator element is low, i.e.in an ideal case, where there are no losses in the element, theresonator element functions like a short circuit. At the parallel oranti-resonance frequency f_(A), respectively, the impedance of theresonator element is high, i.e. in an ideal case without losses theimpedance is infinite and the device resembles an open circuit at theanti-resonance frequency. Therefore, the resonance- and anti-resonancefrequencies (f_(R) and f_(A)) are important design parameters in filterdesign. The resonance and anti-resonance frequencies are determined byprocess parameters like the thickness of the piezoelectric layer of eachresonator element and/or the amount of massloading.

A first known filter type with BAW resonator elements is constructed ina topology known as ladder type topology. For the purposes of thisdescription, ladder type filters that are built primarily of BAWresonator elements are referred to as “BAW ladder filters”. BAW ladderfilters are typically made so that one or more BAW resonators areseries-connected within the filter and one or more BAW resonators areshunt-connected within the filter. Further, a BAW ladder filter istypically designed so that series-connected resonators also called“series resonators”, yield series resonance at a frequency that isapproximately equal to, or near, the desired, i.e. design or center,frequency of the respective filter. Further, shunt-connected resonators,also referred to as “shunt resonators” or “parallel resonators”, yieldparallel resonance at a frequency that is approximately equal to, ornear, the desired center frequency of the respective filter.

A second known circuit topology for filters is the BAW lattice circuit,which circuit topology is also called balanced bridge design. Such a BAWlattice circuit has a stopband when all branches have approximatelyequal impedance and a passband when one branch type, i.e. the series armor the lattice arm, respectively, behaves inductive and the othercapacitive. FIG. 4 shows the impedance characteristics of two differentBAW resonator elements, BAW-1 and BAW-2, usually used in filter design.BAW-1 and BAW-2 are made such, as anti-resonance frequency f_(A1) ofBAW-1 is substantially equal to resonance frequency f_(R2) of BAW-2.Thus, it can be seen that with such two types of BAW resonatorsaccording to the afore-mentioned circuit topologies BAW resonatorfilters can be constructed, which have a passband approximatelycorresponding to the difference Δf between the lowest resonancefrequency, here f_(R1), and the highest anti-resonance frequency, heref_(A2). BAW series and lattice resonator elements may be exchangedprovided series or horizontal resonators are of one type and lattice ordiagonal resonators are of the other type. The bandwidth, i.e. thepassband, of the thus created filter corresponds approximately to thedifference between the highest anti-resonance frequency and the lowestresonance frequency of the used resonator elements. BAW lattice circuitshave the advantage that there is a deep stopband rejection far away fromthe passband.

However, for receive band filters corresponding with moderntelecommunication standards, e.g. PCS, steep transition from stopband topassband is required since Tx and Rx bands are closely separated. Forinstance, in Extended GSM (EGSM) the standard for European secondgeneration 1 GHz mobile communication, Rx and Tx bands are centered at942.5 and 897.5 MHz, respectively. Both bands have a bandwidth of 35MHz, resulting in fractional bandwidth of 3.71% and 3.9% for the Rx andTx, respectively. Moreover, some newer applications, for example, GPS orTV up conversion filter require even smaller bandwidths. A way to reducefilter bandwidth of BAW lattice filters is to decrease the couplingcoefficient of the BAW resonators. However, two types of resonators areneeded, which must provide a slight offset in resonance frequency andanti-resonance frequency. In other words: the narrower needed filterbandwidth, the smaller the needed offset in resonance frequenciesbetween the two types of resonators and thus, the more difficult becomesfabrication process control.

Accordingly, in US laid open patent application U.S. 2001/0012237 asurface acoustic wave (SAW) filter structure includes a piezoelectricsubstrate and a plurality of SAW resonators mounted on the piezoelectricsubstrate. The SAW resonators are connected so as to constitute alattice-type filter circuit having series-arm resonators and lattice-armresonators, wherein the resonance frequency of the lattice-armresonators is substantially equal to the anti-resonance frequency of theseries-arm resonators or vice versa For improvement of such filter inU.S. 2001/0012237 it is suggested to make a gamma value of thelattice-arm resonators and a gamma-value of the series-arm resonatorsdifferent from each other by using capacitors in parallel to the SAWresonators and/or by adjusting SAW resonator geometry.

Therefore, it is an object of the present invention to provide a filterstructure, in particular a radio frequency (RF) filter structure, havinga very narrow passband. It is a further object of the invention toprovide a radio frequency (RF) filter structure, of which thefabrication process can easier be controlled.

Accordingly, a resonator filter structure of the present invention ischaracterized by having resonator elements in a lattice type filtersection having substantially equal resonance frequencies, i.e. fX1R≈fX2Rand substantially equal anti-resonance frequencies, i.e. fX1A≈fX2A. Inother words: the resonator elements of the lattice type filter sectionare made substantially equal, namely the resonance frequency and theanti-resonance frequency of the lattice arm resonator elements and theseries arm resonator elements are substantially equal. Further, thereare means for moving at least one of said anti-resonance frequenciesf_(X1A) and f_(X2A), respectively, or one of said resonance frequenciesf_(X1R) and f_(X2R), respectively, of the resonator elements in one ofsaid lattice branch types.

In a first embodiment of the invention, by connecting discretecapacitances C according to the present invention in parallel acrosseach of both lattice arm resonator elements or across each of bothseries arm resonator elements the anti-resonance frequency of thoseresonator elements is moved, i.e. decreased, by the influence of therespective parallel capacitance. This advantageously results in a verynarrow passband of the whole resonator lattice circuit. It should benoted that series and lattice branches within such a resonator latticefilter section according the invention can be exchanged. In other words:it has the same advantageous effect when the anti-resonance frequency ofthe lattice branches is decreased compared to the anti-resonancefrequency in the series branches as when it is done vice versa.

In a second embodiment of the invention, by connecting discretecapacitances C according to the present invention in series to each ofboth lattice arm resonator elements or in series to each of both seriesarm resonator elements the resonance frequency of those resonatorelements is moved, i.e. increased, by the influence of the respectiveseries capacitance. This advantageously results in a very narrowpassband of the whole resonator lattice circuit. It should be noted thatseries and lattice branches within such a resonator lattice filtersection according the invention can be exchanged. In other words: it hasthe same advantageous effect when the resonance frequency of the latticebranches is increased compared to the resonance frequency in the seriesbranches as when it is done vice versa.

Due to the implementation of a lattice filter section, unbalanced-in tobalanced-out signal guidance can be provided. Thus, one of the inputport and output port can be used as a balanced signal port while theother of the input port and output port is an unbalanced signal port.This is advantageous, when balanced output is preferred, for instance,in case the filter has to be connected to the balanced input of a lownoise amplifier (LNA). Hence, input port or output port of the filterstructure or even none of both may be connected to a fixed referencepotential, e.g. ground potential of the circuit.

By the lattice filter structure of the present invention a very narrowfilter bandwidth can be realized by using one type of resonator in thelattice circuit having one resonance frequency. As resonator elementsare used acoustic wave resonator elements, preferably surface acousticwave (SAW) resonators and more preferably bulk acoustic wave (BAW)resonators. Therefore, the processing is significantly simplified byeliminating the step of creating an offset in resonance frequencies ofthe resonator elements. For example, when BAW resonator elements areused in the invention, such a BAW resonator comprises a stack on asubstrate with at least one or more acoustic reflective layer, a bottomelectrode, a bulk, a top electrode, and an optional massload on top ofthe top electrode. Thereby, the bulk of the BAW resonator elementscomprise a piezoelectric layer having a predetermined thickness andbeing made of an piezoelectric material such as aluminum nitride (AlN)or zinc oxide (ZnO) and having an optional additional dielectric layer,for instance, silicon oxide (SiO2). The combination of silicon oxide(SiO2) and aluminum nitride (AlN) in the BAW resonators reduces thecoupling coefficient of the BAW resonator elements, as required in someapplications with respect, for instance, to bandwidth or temperaturestability. According to the fabrication process of such BAW resonatorelements, advantageously, the thickness of the component layers of thebulk, and/or the massload, and/or the electrode layers for each BAWresonator element can be used to arrange the BAW resonator elements tohave a predetermined resonance frequency and a predeterminedanti-resonance frequency. Further, advantageously all BAW resonators canbe made with same thickness of the piezoelectric layer and furthermore,no massloading is required.

For so-called thickness modes the frequency of acoustic vibration isapproximately inversely proportional to the thickness of thepiezoelectric layer. The piezoelectric thickness is therefore of theorder of 1 micron, so typically a thin-film semiconductor process isused. In one embodiment, the solidly-mounted bulk acoustic waveresonator (sometimes called SBAR) one or more acoustic layers areemployed between the piezoelectric layer and the substrate. Analternative embodiment of thin-film BAW resonator elements (sometimescalled FBAR) employs a membrane approach with the metal-piezo-metalsandwich suspended in air. BAW resonators are often employed in bandpassfilters having various topologies. One advantage of BAW resonators isthe intrinsically better power handling compared to the interdigitatedstructures used in surface acoustic wave (SAW) resonators, especially atfrequencies of modern wireless systems where the pitch of theinterdigital structures must be sub-micron.

With the parallel capacitances C, which are connected in parallel to theresonator elements of one branch type of the lattice type filtersection, i.e. the series or the lattice arm resonator elements, theanti-resonance frequencies of those resonators are decreased. Thus, thecapacitance value C is used for tuning the filter bandwidth, wherein thesmaller the capacitance C, the smaller the resulting bandwidth of thefilter. The inventors have found that to create a good stopbandrejection, the total capacitances of each branch need to be equal in thestopband. Therefore, good stopband rejection, i.e. attenuation, has beenachieved when the value of the parallel capacitances C correspondsubstantially withC=(1−x)·A·C _(AREA).In this equation A is an area of a BAW resonators on the substrate,where the BAW resonator element is mounted to, of one of the branchtypes of the lattice type filter. Further the area on the substrate hasa capacitance per area C_(AREA). The factor x is a fraction of the areaA, wherein x·A is the area of a BAW resonators of the respective otherbranch type of the lattice type filter. Such capacitance C has to beconnected as parallel capacitance in parallel to each BAW resonatorhaving area x·A on the substrate of the device.

As to practical implementation of the BAW lattice filter design of theinvention, a BAW resonator basically consists of two electrodes with apiezoelectric material enclosed between these two electrodes. In turnthis whole system is enclosed between to acoustically reflecting mediain order to keep the acoustic energy in the piezoelectric layer. AUlayers will impact the resonator properties, e.g. resonance frequency,bandwidth, Q-factor etc. In order to turn lattice circuits and cascadedlattice and ladder circuits into commercially attractive products, it isalmost a prerequisite to have two electrodes. These two electrodes,inherent to BAW resonators, provide a solution for the needed crossingof interconnected lines. In contrary to BAW elements, electrodes of SAWresonator elements are typically defined in one metal layer thereforemaking lattice interconnect wiring much more complicated.

As to the means for impedance matching, according to the invention thereare inductive elements L and/or capacitive elements C, which areconnected in series and/or parallel to the one or both of the input portand output port. In one embodiment the means for impedance matchingcomprise a series capacitive element connected in series to one of theports and series capacitive element connected in series to the other ofthe ports. It should be noted that the inventors have discovered that incertain cases there are no discrete impedance matching means necessary,or impedance matching has to be done only at one port of the input portor output port.

The above and other objectives, features and advantages of the presentinvention will become more clear from the following description of thepreferred embodiments thereof, taken in conjunction with theaccompanying drawings. It is noted that through the drawings same orequivalent parts remain the same reference number. All drawings areintended to illustrate some aspects and embodiments of the presentinvention. Circuits are depicted in a simplified way for reason ofclarity. It goes without saying that not all alternatives and optionsare shown and therefore, the present invention is not limited to thecontent of the accompanying drawings.

In the following, the present invention will be described in greaterdetail by way of example with reference to the accompanying drawings, inwhich

FIG. 1 shows a circuit diagram of a resonator filter structure with BAWlattice filter section arranged with parallel capacitances connected inparallel to lattice arm BAW resonators and impedance matchingcapacitances connected in series to the ports of the Filter;

FIG. 2 illustrates an equivalent element circuit of a BAW resonatorelement together with a parallel capacitance connected in parallel tothe BAW resonator;

FIG. 3 shows impedance characteristics of two substantial equal BAWresonator elements A and B, wherein the effect of a parallel capacitanceas shown in FIG. 2 on the anti-resonance frequency of a BAW resonator Bis depicted by a doted line; and

FIG. 4 shows the impedance characteristics of two BAW resonator elementsdrawn over signal frequency, wherein resonance frequencies,anti-resonance frequencies and center frequency are arranged as usual.

FIG. 1 shows a resonator filter structure 10 according to the firstembodiment of the present invention that comprises a first port 1 and asecond port 2. There is connected a first load 3 to the first port 1 anda second load 4 towards the second port 2. The first port 1 has aconnection to a fixed reference potential which is in this exampleground 5 of the circuit. Thus the input port 1 is a port with unbalancedsignal guidance. The first load may represent an internal resistance ofa generator that is driving a radio frequency signal as input for theresonator filter structure 10; in an application such generator, forinstance, may be a receiving antenna of a communication unit. Further,the second load 4 may represent the input resistance of a followingstage like, for instance, a low noise amplifier (LNA). As to signalguidance the first port 1 is grounded to ground 5, which causes thefirst port 1 to be an unbalanced port.

It is clear for the man skilled in the art that for the reason ofoptimal power transition there is need for impedance matching at theinput port 1 and the output port 2. At least, the input impedances ofthe resonator filter structure 10 have to be matched according to therespective loads 3 and 4 within the frequency band that corresponds tothe passband of the resonator filter structure 10. The passband isdefined by a lower cut-off frequency, a center frequency, and an uppercut-off frequency, wherein a cut-off frequency could be derived by acertain signal power level to which the signal has decreased frompassband towards the stopband.

The central part of the shown resonator filter structure 10 in FIG. 1 isa lattice filter section 20, which comprises four substantial equal BAWresonator elements 20-1, 20-2, 20-3, 204. The circuit structure, i.e.topology, of this BAW lattice filter section 20 is constructed with thefour BAW elements 20-1, 20-2, 20-3, 20-4 in the known principle of abalanced bridge circuit. Thus, respective two of the four resonatorelements, i.e. 20-1 and 20-2 are connected in series building a firstseries path, and 20-3 and 20-4 are connected in series building a secondseries path. The connection nodes between two BAW elements of the firstand second series path represent respective one output node of the BAWlattice circuit. Further, first and second series path of the bridge areconnected in parallel to the input nodes of the BAW lattice circuit. Dueto the illustration of the BAW lattice filter section 20, resonatorelements 20-1 and 20-4 are also called horizontal elements or serieselements of the BAW lattice circuit, and BAW elements 20-2 and 20-3 arealso called diagonal elements or lattice elements of the BAW latticefilter section 20. Moreover, according to this naming convention eachbranch of the BAW lattice filter section 20 is called an arm of thelattice circuit, wherein a horizontal element builds an horizontal orseries arm, respectively, and a diagonal element builds a diagonal orlattice arm of the lattice circuit, respectively.

In FIG. 1, all BAW resonators 20-1, 20-2, 20-3, 204 of the BAW latticefilter section 20 are substantial equal that means the individual BAWelements have same resonance frequency and same anti-resonancefrequency. However, between the horizontal BAW resonator elements 20-1,204 and the diagonal BAW resonator elements 20-2, 20-3 there is adifference in area on the substrate on which the filter circuit ismanufactured. The parallel capacitances 30-1, 30-2 which are connectedin parallel towards the diagonal BAW elements 20-2, 20-3 move theanti-resonance frequencies of respective diagonal BAW resonators 20-2and 20-3. In other words, as tuning elements the capacitances 30-1 and30-2 adjust the filter passband and thus, ease advantageouslyfabrication process of the BAW lattice filter section 20 according tothe present invention.

Oppose horizontal BAW resonators 20-1 and 20-4 have an area A anddiagonal BAW resonators 20-2 and 20-3 have a fraction x of this area Aon the substrate, wherein x is a value between zero and 1. Theelectrodes of the BAW resonators cause a capacitance per area C_(AREA).As to an optimal value for the parallel capacitances 30, it has found bythe inventors that for a good stopband rejection the total capacitanceof each lattice arm in the BAW lattice type filter section 20 needs tobe equal. Thus, taking the values for the parallel capacitances 30-1,30-2 according to the formulaC=(1−x)·A·C _(AREA)provides for good stopband rejection. It goes without saying that forachieving the same effect on the bandwidth horizontal, i.e. series, armBAWs may be swapped with diagonal, i.e. lattice, arm BAWs. The BAWlattice filter section 20 has its desired center or design frequencywhich lays approximately in the middle between the anti-resonancefrequency of the series arm BAW resonator elements 20-1 and 20-4 and thelattice arm BAW resonator elements 20-2 and 20-3.

As to the impedance matching for improved power transition, at the inputport 1 of the filter structure 10, there are impedance matching sections40 a and 40 b. Impedance matching section 40 a comprises a seriescapacitance 41 which is arranged for impedance matching between theinput impedance of the BAW lattice filter section 20 and the outputimpedance of the load 3. At the output port 2 of the BAW lattice filtersection 20, there is a impedance matching section 40 b with two seriescapacitances 42-1 and 42-2 connected symmetrically, for the reason ofbalanced signal guidance, in series to the output port 2, which isarranged for impedance matching between the output impedance of the BAWlattice filter 10 and the input impedance of the load 4.

Since all BAW lattice elements 20-1, 20-2, 20-3, 20-4 have equalresonance frequency and anti-resonance frequency, which is very unusualin lattice filter technology, the fabrication process is advantageouslysimplified by eliminating the step of creating an offset in resonancefrequencies. This means all resonators can be made with the samethickness in the piezoelectric layer, furthermore no massloading isrequired.

FIG. 2 shows the equivalent element circuit for a BAW resonator element50, which comprises a series inductance L_(S), a series capacitanceC_(S) and a series resistance R_(S). The resonance frequency f_(R)corresponds to L_(S) and C_(S). Further, there is a parallel capacitanceC_(P) that is mainly caused by the electrodes of the BAW resonator. Theanti-resonance frequency of the BAW resonator element 50 corresponds toL_(S) and both capacitances C_(S) and C_(P). Moreover, there is aparallel capacitance 30, which is drawn in doted lines, being connectedin parallel to the equivalent elements of the BAW resonator 50. With theparallel capacitance 30 it is possible to tune the anti-resonancefrequency by increasing the effective parallel capacitance, which iscomposed substantially of C_(P) and C.

FIG. 3 is a diagram of the impedance characteristic of the BAW resonatorelements of the BAW lattice filter structure according to the presentinvention. More clearly, there is a first impedance characteristic drawnin solid line of a BAW resonator A which corresponds to those BAWresonators without a parallel capacitance C which are in FIG. 1 the BAWresonators 20-1 and 204. The second impedance characteristic drawn inbroken line of a BAW resonator B corresponds to those BAW resonatorsWith parallel capacitance C which are in FIG. 1 the BAW resonators 20-2and 20-3. From FIG. 3 can easily be seen that both types of BAWresonators have equal resonance frequency f_(R). However, there is aslight difference between the anti-resonance frequencies f_(A) of BAWresonator A and resulting anti-resonance frequency f_(A*) of BAWresonator B which is designed by use of the parallel capacitances Cwhich is represented in FIG. 1 by the capacitances 30-1 and 30-2 and inFIG. 2 by the capacitance 30.

With respect to FIG. 3 it can be understood how the BAW lattice filtersection 10 can provide such very narrow passband. The BAW lattice filtersection 10 of the invention has a stopband when all branches have anequal impedance and a passband when one branch type, i.e. the series armor the lattice arm, respectively, behaves inductive and the othercapacitive. Due to the fact that there are used BAW resonatorsresonating at approximately at the same frequencies, i.e. resonancefrequency f_(R) and anti-resonance frequency f_(A), in all branches ofthe BAW ice filter section 20, while the anti-resonance frequency ofboth BAW resonators in one branch type of the BAW lattice filter section20, i.e. the series arm or the lattice arm, respectively, is moved justa bit, there is only a small frequency range Δf were one branch type isinductive and the other capacitive. This way, the very narrow bandpassfilter is created, wherein the passband is much narrower than thedifference between resonance frequency f_(R) and anti-resonancefrequency f_(A) of one single BAW resonator. The caused passband can bedetermined approximately by the difference in anti-resonance frequenciesf_(A) and f_(A*) between the lattice and the series branch BAWresonators which is depicted as Δf in FIG. 3. It goes without sayingthat the BAW series and lattice resonator elements may be exchanged.

With the present invention a resonator filter structure for a radiofrequency (RF) filter has been introduced, especially a bulk acousticwave (BAW) filter structure. According to the invention, a resonatorfilter is constructed with a BAW lattice filter configuration, in whichall of the BAW resonator elements within the BAW lattice filter sectionhave substantially equal resonance frequencies. According to theinvention, there are parallel capacitances connected in parallel to theBAW resonators of one branch type of the BAW lattice filter section.Thus, anti-resonance frequency of the respective BAW resonator is tuned.That results in a very narrow passband which corresponds approximatelyto the difference in anti-resonance frequencies between diagonal andhorizontal branches of the lattice filter section. The parallelcapacitances are used to tune the bandwidth: the smaller thecapacitance, the smaller the bandwidth. Moreover, due to the latticestructure at one port of the resonator filter signal guidance will bebalanced while at the other port signal guidance can be unbalanced orbalanced, as needed in the respective application.

It should be noted that the present invention is not restricted to theembodiments of the present invention, in particular the invention is notrestricted to receive filters which have been used in this specificationfor reason of example. Moreover, the principle of the present inventioncan be applied to any application that needs in a high frequencyenvironment a filter that provides very narrow bandwidth and highstopband.

1. A resonator filter structure (10), in particular a radio frequency(RF) filter structure, arranged on a substrate for providing a passbandwhich can be defined by frequencies as a center frequency f_(C), a lowercut off frequency f_(L), a upper cut off frequency f_(U) comprisingbetween an input port (1) and an output port (2) at least a lattice typefilter section (20) having two lattice branch types being a latticebranch and a series branch wherein resonator elements (20-1, 20-2, 20-3,20-4) are arranged in said series branches as series arms having aresonance frequency f_(X1R) and an anti-resonance frequency f_(X1A) andwherein resonator elements (20-1, 20-2, 20-3, 20-4) are arranged in saidlattice branches as lattice arms having a resonance frequency f_(X2R)and a anti-resonance frequency f_(X2A), characterized in that all ofsaid resonator elements (20-1, 20-2, 20-3, 20-4) within said latticetype filter section (20) have substantially equal resonance frequencies,i.e. f_(X1R)≈f_(X2R), and substantially equal anti-resonancefrequencies, i.e. f_(X1A)≈f_(X2A); and there are means (30-1, 30-2) formoving at least one of said anti-resonance frequencies f_(X1A) orf_(X2A), respectively, or one of said resonance frequencies f_(X1R) andf_(X2R), respectively, of the resonator elements in one of said latticebranch types.
 2. The resonator filter structure according to claim 1,wherein said means for moving at least one of said anti-resonancefrequencies f_(X1A) or f_(X2A), respectively, are parallel capacitancesC (30-1, 30-2) connected in parallel to each of said series armresonators or to each of said lattice arm resonators (20-2, 20-3),respectively.
 3. The resonator filter structure according to claim 1,wherein said means for moving at least one of said resonance frequenciesf_(X1R) or f_(X2R), respectively, are series capacitances C connected inseries to each of said series arm resonators or to each of said latticearm resonators, respectively.
 4. The resonator filter structureaccording to claim 1, wherein said resonator elements (20-1, 20-2, 20-3,20-4) are acoustic wave resonator elements, preferably bulk acousticwave (BAW) resonator elements or surface acoustic wave (SAW) resonatorelements.
 5. The resonator filter structure according to claim 4,wherein said BAW resonator elements comprise at least a piezoelectriclayer having an equal thickness in all of said BAW resonator elements.6. The resonator filter structure according to claim 5, wherein saidpiezoelectric layer comprises a layer of piezoelectric material such asaluminum nitride (AlN) and/or zinc oxide (ZnO) and at least an optionaladditional dielectric layer such as silicon oxide (SiO₂).
 7. Theresonator filter structure according to claim 1, wherein a totalcapacitance of all branches of said lattice type filter section issubstantial equal at least outside said passband.
 8. The resonatorfilter structure according to claim 7, wherein said parallelcapacitances C (30-1, 30-2) corresponds withC=(1−x)·A·C _(AREA), wherein A is an area on said substrate of one ofsaid BAW resonators in one of said branch types of said BAW latticefilter section wherein said area has a capacitance per area C_(AREA) andx is an fraction of said area A, wherein x·A is an area of one of saidBAW resonators of said respective other branch type of said BAW latticefilter section.
 9. The resonator filter structure according to claim 1,wherein means for impedance matching (40 a, 40 b) are connected at leastto one of said input port and said output port.
 10. The resonator filterstructure according to claim 8, wherein said means for impedancematching (40 a, 40 b) comprise discrete inductive elements and/ordiscrete capacitive elements (41, 42-1, 42-2) which are connected inseries and/or in parallel to said respective port.
 11. The resonatorfilter structure according to claim 1, wherein at least at one of saidinput port and output port signal guidance is balanced.
 12. Theresonator filter structure according to claim 11, wherein at one of saidinput port and output port signal guidance is unbalanced.